The present invention relates to a pneumatic radial tire for a heavy load.
As shown in FIGS. 4 and 6, a pneumatic radial tire generally includes at least one layer of carcass ply 1 constituted by an array of cords in the radial direction thereof, and both ends of this carcass ply 1 are anchored at bead portions 2 on both sides by being upwardly wound around bead cores 3 and rubber fillers 4 provided thereon from the inside to the outside thereof. Further, the anchoring portions are reinforced by providing them with bead portion reinforcement layers 5 called chafers having cords made of steel or organic fiber.
One or a plurality of belt layers 8 are provided outside the carcass ply 1 at a tread portion 7, and rubber layers such as side walls and tread rubber are deposited and molded on the exterior of the carcass ply 1 and the belt layer 8 to be integral therewith, thereby forming a tire. Reference numeral 9 designates main grooves provided on a circumferential surface of the tread portion 7.
As the above-described rubber fillers 4 of the bead portions 2, not only a rubber filler consisting of only one kind of rubber is used as shown in FIG. 6, but also a rubber filler 4 which is a combination of a hard rubber filler and a soft rubber filler has been proposed (for example, see JP-A-02133208 and JP-A-06064412 (Japanese unexamined patent publications No. H2-133208 and No. H6-64412)).
A tire having the above-described structure has the following problems.
When the bead cores 3 are constituted by a collection of bead wires having a circular sectional configuration, point contact occurs between the wires as the carcass ply 1 produces a moment that causes the inner ends of the bead cores to rotate outward in the radial direction of the tire, under a pressure during the application of an internal pressure and load.
When the point contact occurs, the contact points between the bead wires act like a fulcrum of a lever to move the bead cores upward. A geometrical moment of inertia of the bead cores during such rotation is very weak relative to the strength of the tire in the circumferential direction thereof. It is therefore difficult to maintain the configuration of the beads. Further, while a high tension acts on inner wires which are moved upward, the tension of outer wires is significantly reduced. Therefore, one cannot expect a uniform distribution of stress in the bead cores that is essential to maintain the configuration of the same.
In this regard, a distribution of stress in a section of a bead core having a hexagonal sectional configuration will be described with reference to a truss structure shown in FIG. 7.
A compression stress is developed in an upper outer layer of a bead core 3 due to the tension of a carcass ply in the radial direction of the tire that acts on the inner end of the bead core (the end at the inner side of the tire), and this force is dispersed by point contact between outer wires 30. A moment produced by the contact points between the outer wires acting as a fulcrum of a lever moves the inner end of the bead core upward. As a result, a tensile stress is primarily developed inside the bead core. Further, the tension of the carcass ply is concentrated on the wires that are moved upward in the vicinity of the inner end of the core rather than being dispersed. Thus, the bead core is deformed.
The deformation of the bead core 3 results in a reduction in the width of the base of the bead core 3 and an upward displacement of a bead toe portion 2a (indicated by the chain line in FIG. 6). This degrades air sealing between rim flanges 10 and the tire beads, thereby making an air charging operation difficult. Meanwhile, the upward displacement of the inner ends of the bead cores that serve as anchoring points for the carcass ply 1 results in a change of the periphery of carcass lines on the inner surface of the tire. An upward displacement of the ply anchoring points at the inner ends of the bead cores results in an upward displacement of the starting point of the carcass ply, which means an increase in the substantial periphery of the tire. However, it is generally known that the entire width of a tire tends to remain unchanged or decrease when the substantial periphery of the tire is increased. Therefore, a periphery that is in excess must be absorbed only by growth of the outer diameter (see FIG. 8).
Further, a change in the configuration of the inner surface of bead portions as a result of outward bulging deformation of the same after a run results in the same effect as described above and hence the rigidity of the bead portions has influence on a tread crown portion. For example, when the bead portions are deformed so as to bulge outwardly as a result of growth of the outer diameter of the tire after a run relative to the bead configuration before the growth of the outer diameter as indicated by the chain line in FIG. 6, such bulging can cause an upward displacement of the bead toe portion 2a which leads to an excess periphery as described above.
The absorption of an excess periphery by the growth of the outer diameter as described above is supplemented by growth of the regions at both ends of the belt layer 8 which have a low hooping effect and low rigidity. As shown in FIG. 4, the both ends of the belt layer 8 are located in the vicinity of points G aparted from ends of a ground contact of the tire by a quarter of a width of the ground contact (hereinafter referred to as quarter points) and is substantially aligned with the outermost main grooves 9. Since the rigidity of the wheel tread of the tire is lowest at the regions of the main grooves 9, growth of the outer diameter occurs at the main grooves 9 in the vicinity of the above-described quarter points as starting points. For example, the tire tread crown bulges as shown in FIG. 5. This takes the form of protrusions W1 in the vicinity of the quarter points G of the tread crown as encountered on a tire after growth and is a phenomenon that is more significant on a low aspect tire. Further, the above-described protrusions W1 in the vicinity of the quarter points G can cause the formation of a crown configuration having sloping shoulders which is susceptible to irregular wear abrasion.
Meanwhile, a conventional rubber filler which is a combination of a hard rubber filler and a soft rubber filler is intended for suppressing only the movement of bead cores, or intended for improving only the durability of bead portions by suppressing inclination of the same, and is not directed to prevention of protrusion arised at quarter points of a tread crown. Therefore, it has not be satisfactory in preventing such protrusions W1.
The present invention has been conceived taking the above-described points into consideration, and it is an object of the invention to suppress upward displacements of bead toe portions 2a and carcass ply anchoring points as a result of outward bulging deformation of bead portions by the use of hard rubber fillers 4a at above bead cores 3, thereby preventing protruding deformation W1 in the vicinity of quarter points G as a result of growth of the outer diameter of a tread crown and further preventing an increase in rolling resistance and excess heat generation.